Understanding the mechanisms that generate and maintain cell polarity is a fundamental challenge of cell biology. The broad goal of this proposal is to understand the mechanism by which oskar mRNA is localized to the posterior pole of the Drosophila oocyte. This localization is dependent on an extracellular signal that polarizes the oocyte's microtubule cytoskeleton, an intact actin cytoskeleton, and Rab 11, a member of the Rab family of Ras-like GTPases, which mediate vesicle traffic in the endocytic recycling pathways of all examined organisms from yeast to human. While the roles of microtubules and actin filaments in oskar mRNA localization are well established--they provide the tracks along which oskar mRNA is transported and a substrate for anchoring--the role of Rab 11 and vesicle trafficking is poorly understood and is the focus of the proposed work. In rab11 mutants, oskar mRNA accumulates near, rather than at, the oocyte's posterior pole and is not translated. The following hypotheses could explain the requirement for rab 11 in oskar mRNA localization: a) Rab 11 protein maintains extracellular signaling (and thus MT polarization) by recycling key receptor molecules back to the oocyte's plasma membrane, b) Rab 11 mediates short-range transport of oskar mRNA from MTs to actin filaments by recruiting actin-based motor proteins to oskar mRNA transport complexes, and c) Rab 11 maintains the organization of cortical actin filaments by repairing damage associated with active endocytosis. To test these hypotheses, we will determine which aspect or aspects of the oskar mRNA localization pathway, i.e., extracellular signaling/MT organization, oskar mRNA transport and anchoring, and/or organization of the actin cytoskeleton, are most sensitive to a reduction in rab11 gene activity (Aim 1). This will be accomplished through pair-wise reductions in the activity of rab11 and other genes in the oskar mRNA localization pathway. In addition, domain swapping and site-directed mutagenesis will be carried out to determine which parts of Rab 11 are required for oskar mRNA localization (Aim 2). Finally, genetic and biochemical strategies will be used to identify and characterize proteins that bind to Rab 11 (Aim 3). The results of the proposed studies should elucidate the mechanism by which Rab 11 mediates oskar mRNA localization and, through an increased understanding of membrane trafficking, may reveal new targets for therapeutic intervention in diseased cells.